Synchronizing circuit



Aug. 22, 1944. A. R. APPLEGARTH, JR

SYNCHRONIZING CIRCUIT Filed July 51, 1942 3 Sheets-Shea*I l rp Q Y NRSRGMQ Aug 22, 1944. A. R. APPLEGARTH, JR

SYNCHRONIZING CIRCUIT Filed July 31, 1942 3 Sheecs-SheerI 2 @www 5Sheets-Sheet 3 A. R. APPLEGARTH, JR

SYNCHRONIZING CIRCUIT Filed July 5l, 1942 htm ms GPM@ Patented Aug. 22,1944 SYNCHRONIZING CIRCUIT Alexander R. Applegarth, Jr., Dayton, Ohio,assignor to Philco Radio and Television Corporation, Philadelphia, Pa.,a corporation of Dela- Ware Application July 31, 1942, Serial No.453,138

23 Claims.

'I'his invention relates to certain improvements in synchronizingcircuits and more particularly synchronizing circuits of the typedesigned to be actuated by pulse signals, such for example as thoseemployed in television receivers. As is well known, television receiversmust be operated in synchronism with the transmitting equipment fromwhich the received signal emanates. For this purpose it is customary atthe transmitter periodically to reduce the video signal to apredetermined amplitude level and to superimpose during such intervalsof reduction, in the opposite polarity from that of the video signal,pulse signals of substantially constant amplitude. The resulting signalmay be caused to modulate a carrier wave of suitable frequency so thatthe signal may be radiated.

At. the receiver this signal may be detected and ampliiied by any of thewell known means suitable for this purpose. It then becomes necessary toseparate the synchronizing portion of the signal from the video portionin order that the former may be used to control the operation ofdeflecting circuits used in conjunction with cathode ray devices toreconstitute the visual image thus electrically transmitted. Thisseparation must be performed with considerable care in order that goodsynchronizing may obtain. This raises many problems which must be takeninto consideration in the design of equipment for eiecting thisseparation. For example it is essential that the selected portion shouldinclude only the synchronizing pulses to the exclusion of the videosignal and to the substantial exclusion of noise pulses of largeamplitude. This separation process is rendered diilicult by the factthat the strength of the received signal may not be uniform due tovarying attenuation of the signal in transmission and also by the factthat,

for.` various reasons, the received synchronizing pulses may not alloccur at the same amplitude level. It is further desirable that thepulses in the selected synchronizing signal should be of substantiallyconstant amplitude.

The principal object of this invention is to provide means forovercoming these difficulties and for yielding a high qualitysynchronizing signal of substantially constant amplitude andsubstantially free from any video signal and large amplitude noisepulses. The details of the invention will be clearly understood from thefollowing description with reference to the accompanying drawings, inwhich Fig. 1 is a diagrammatic illustration of one embodiment of theinvention; I

(Cl. P18-7.5)

Fig. 2 is an explanatory diagram which will be referred to in explainingthe operation of the invention; and

Fig. 3 is a diagrammatic illustration of another embodiment of theinvention.

In Fig. 1 there is represented generally in block I those parts of thereceiver which are conventional and unmodified by the invention. Thesecircuits may include a radio frequency amplifier supplied with signalfrom antenna 2, a converter stage, and an intermediate frequencyamplifier. As shown the iinal stage of the latter may be coupled bymeans of the transformer 3 to a detector circuit comprising the diode 6,resistor 5, condenser 6, and chokes 'I and 8. The detected signaldevelops across the load impedance comprising the resistor 5 and thepeaking coil 8. Filtering to remove the carrier frequency is effected bythe combination of the condenser 6 and the choke 1. The detected signalis applied to the grid of the amplifier tube 9 which may be a pentode.Suitable biasing for this tube is obtained across the voltage divider I0by-passed by the condenser I I. The tube may be degenerated slightly bythe inclusion in its cathode circuit of a small resistor I2 for thepurpose of lengthening out the grid voltage-plate current characteristicso that the signal applied to the grid of the tube will not be limitedby tube cutoil. Two separate output circuits may be provided for thetube 9, one in the anode circuit comprising the resistor I3 and thepeaking coil I4, and another in the 'screen grid circuit comprising theresistor I5 and the peaking coil I6. Across a portion of the resistanceof the potentiometer I1 shunting the anode load may be taken a signalwhich is fed to the video amplifier I8 and applied to the grid of thepicture tube I9. Across the screen load may be taken the signal fromwhich there is separated out the synchronizing component for controllingthe oper-ation of the synchronizing circuits 20 which supply adeflecting signal to the deflecting coil 2l for defleeting the electronbeam in the picture tube. For the sake of simplicity a single deflectingcoil has been shown, though it is of course to be understood that theboth horizontal and vertical deflecting means are employed in the usualtelevisionreceivers. The screen load may be chosen so as to peak at' alower frequency than the plate load. This tends to improve the responsein the video channel while giving a sumciently good response in thesynchronizing channel which does not require as wide a band offrequencies as does the video channel. This feature is disclosed andclaimed in U. S. Patent No. 2,289,291, granted July 7, 1942.

Signal derived across the screen load impedance is supplied through acoupling condenser 22 and across grid-leak 23 to the grid of the tube24. The latter is preferably a vbeam-power tube, though a pentode mightbe used. The tube is permitted to draw grid current on signal peakswhereby it acts in cooperation with the condenser 22 and the grid leak2,3 to develop a varying ybias for leveling the peaks of the signal inthe manner shown at 25. The cathode is maintained at the desiredpotential by connection to the voltage divider comprising resistors 26and 21, the latter of which is by-passed by condenser 28. 'I'he tube isoperated at low anode potential from the divider comprising resistors 29and 30 whereby it is operative to pass only a portion betweenpredetermined amplitude levels of the signal supplied to its input, theamplitude levels being determined respectively by plate current cut-ofi'and plate current saturation. As a result the tube operates on curves ofanode current versus grid voltage as shown in Fig. 2'. Further it willbe noted that the screen of tube 9 derives voltage from a dividercomprising the resistors 3|, 32 and 33, of which 3| is connected betweenthe screen of tube 24 and ground and suitably by-passed by the condenser34 so that variations in the D. C. screen voltage of the tube 9 appearon the screen of tube 24. This variation in the screen voltage iseffective to cause the tube 24 to select a portion of the signalintermediate between the synchronizing tips and the blanking level.

The operation of the circuit will now be considered in somewhat greaterdetail with reference to Figure l and 2. In view of the direct couplingbetween the detector and the grid of tube 9, any variations in signalstrength will appear as variations in the level of the signal applied tothe grid of that tube. For example, at 35 a weak signal is representedby the solid curve and a strong signal by the broken curve. Hence itwill be seen that for weak signals the tube 9 draws a large screen andplate current thereby making its D. C. screen voltage low, while forstrong signals the screen current will be reduced and the screen voltagewill rise.

Considering now the operation of the tube 24, this tube is connected tooperate as a gate capable of selecting a. portion between predeterminedamplitude levels of the signal applied to its grid. Furthermore it ispossible, by varying the screen voltage of this tube with respect to theanode and cathode voltages, to vary the level at which this portion isselected with reference to the signal applied to its input. This vnll beunderstood more clearly by reference to Fig. 2 which shows curves ofplate current versus grid voltage for the tube 24 for various values ofscreen voltage when connected as shown in Fig. 1. 'Ihus for a relativelyhigh screen voltage the tube may operate on the curve labeled E1 whilefor progressively lower screen voltages it may operate upon the curvesE2 and Ea. Since the screen grid of tube 24 is connected so as to besubject to variations in the D. C. screen voltage of the tube 9, whichis dependent upon signal strength,

it will appear that the characteristicupon which f the tube 24 operateswill depend upon signal strength From Fig. 2 it will be apparent thatthe porc and d. Parallel tion of. the signal selected will depend uponthe characteristic upon which the tube operates. Thus when operating oncharacteristic Es the tube will select that portion of the input signalcorresponding to grid voltages between the values of a and 0, while,when operating on the c urve E2, that portion of the signal will beselected corresponding to grid voltage values between to the eg axis inFig. 2 are shown the synchronizing tips and portions of the blankingpedestals of a weak and of a strong signal, designated f and grespectively. The portions of these signals selected by the tube 24 areshaded and the resultant plate current pulses are shown at f and grespectively. In the case of the weak signal f the voltage on the screenof tube 24 will be relatively low so that the tube may operate on thecharacteristic Es selecting that portion of the signal between thevalues a and 0 of grid voltage.v However, in the case of the strongsignal 9' the screen voltage will be larger and Athe tube may operate oncharacteristic E2 selecting a portion between relatively lower values ofgrid voltage.

This mode of operation will be seen to be desirable for several reasons.For weak signals the tube selects substantially the entire synchronizingpulse between its tip and the blanking level, but for stronger signalsonly a fraction of the pulse is required. It then becomes desirable totake this portion intermediate between the usual level of synchronizingtips and the blanking level. The advantage in doing this obtains when,as is frequently the case, the synchronizing tips do not occur at aconstant level. Thus, in the case of the pulse shown at h, if the tubewere operating on the characteristic E3, the grid would not be swungsutiiciently far in the positive direction to give the desired output,the resulting pulse being shown 'by the cross-hatched area at h.However, when operating on the characteristic Ez, the swing, though -notas great as in the case of the pulses f and g, is suflicient to give thedesired output. Likewise it follows 'that this level of selection cannotbe maintained for weak signals Without the inclusion, in the selectedportion, of a portion of the video component which would be veryundesirable.y

It will be observed from the characteristics in Fig. 2 that the gateoperates to select a constant amplitude portion of the signal suppliedto its input, regardless of the level at which the selec"- tion is made.Such operation is conducive t'o' good synchronization and tends..toreduce -the amplitude of any noise pulses which may be present in thesignal. However it is to be noted that this is but a ypreferred mode ofoperation to which the invention is not restricted. It is of courseconceivable that the tube might be used in a circuit such that each ofthe characteristics differs in slope and in'remoteness of cut-oil fromthe others. In such a case. the gate would not only select a portion ofthe signal at a varying level but would also select a portion of varyingmagnitude, which might prove satisfactory in certain applications. Thismay also depend upon the type of tube used, for, although applicant hasfound a beam-rpower tube suited to his requirements, it is obvious thattubes of other types such as pentodes and those having even a greaternumber o1' electrodes might also be used.

'I'he following circuit element values may be taken as exemplary in theembodiment of Fig. 1

but they are not to be regarded as restricting the scope of theinvention in any way:

Tube 9=type 1232 Tube 24=type 6Y6G Resistor I2:56 ohms Resistor I 3:4700ohms Resistor I5:10,000 ohms Resistor 23:1 megohm Resistor 26:100,000ohms Resistor 21:1000 ohms Resistor 29:1(),000 ohms Resistor 30:1 megohmResistor 3|:l2,000 ohms Resistor 32:330,000 ohms Resistor 33=4'7,000ohms Inductor I4=70 microhenries Inductor I6:1000 microhenries Condenser22:.05 microfarad Condenser 23:4 microfarads Condenser l34:10microfarads Referring now to Fig. 3 there is shown another embodiment ofthe invention wherein the control applied to the screen grid of the gatetube is supplemented by the application to the anode and cathode of thetube, of a control voltage varying in the opposite sense (i. e., becomesmore positive with decreasing signal strength). Such a voltage mayconveniently be obtained across an impedance in the cathode circuit ofthe amplifier tube following the detector. It will of course occur tothose skilled in the art that it is possible to derive thislast-mentioned control voltage, as well as the voltage varying in theopposite sense, in various ways and at various points in the system, andit is not intended to limit the invention to the specic means andmethods herein set forth.

In the circuit of Fig. 3 those circuits included in the block designatedby the reference numeral I as well as the video and synchronzingchannels may be the same as in the circuit of Fig. l. However thefunctions of the detector and the succeeding am-plier are here performedby a single diode-pentode 36. The output circuit for this tube mayinclude, in additionto the resistor 31, the peaking coils 38 and 39.Bothvideo and synchronizing outputs are taken across the impedanceconsisting of the resistor 31 and the inductor 39. The synchronizingcomponent is supplied through an isolating resistor 40 and a couplingcondenser 4| across a grid leak 42 to the grid of tube 43 which isoperated at loW plate voltage'obtained from the voltage dividercomprising resistors 44, 45 and 46. As in the circuit of Fig. 1, thetube 36 derives screen voltage from a divider including the resistors41, 48 and 49, of which 41 is connected between the screen of tube 43and ground and suitably by-passed by condenser 50 for the purposealready explained of varying the operating level of the gate.

In certain cases the amount of control thus afforded may beinsuiiicient. Another method of obtaining equivalent control is by theapplication of a control voltage varying in the opposite sense to thecathode and anode of the gate tube. For this purpose the resistor 46forming a, part of the anode voltage divider of tube 43 is included inthe cathode circuit of tube 36 suitably by-passed by condensers 5| and52. By reason of this connection the cathode and anode of tube 43 arecaused to assume a more positive potential as the signal decreases andvice versa.

obtains when the cathode and anode potentials are fixed and the screenvoltage caused to vary in the opposite sense. .These two controllingeffects cooperate to yield the desired variation in the orperating levelof the gate with reference to the input signal. 'i

The following circuit element values are exemplary of those which may beused in the circuit of Fig. '3 but are not to be regarded as restrictiveof scope:

Although the invention has been described with reference to certainspecific embodiments, it will be understood that it is capable ofphysical expression in numerous other forms without departing from itsscope which is subject only to the limitations implosed by the appendedclaims.

I claim:

l. Inan electrical system; a source of a signal of varying strength; aspace discharge device having its input coupled to said source forselecting a portion between predetermined amplitude levels of the signalsupplied to its input, said device being operated at low anode potentialthereby to determine the magnitude of the se- The eilect of this uponthe operation of the gate g lected portion, and having an auxiliarycontrol element, the potential of which determines the level at whichsaid portion is selected; means for deriving a control voltage whichvaries with said signal strength; and means for applying said voltage tosaid control element to control the level of selection..

2. In an electrical system; a source of a signal of varying strength; aspace discharge device having its input coupled to said source forselecting a portion between predetermined amplitude levels of the signalsupplied to its input, said device being operated at low anode potentialthereby to determine the magnitude of the selected portion, and havingan auxiliary control element, the potential of which determines thelevel at which said portion is selected; means for deriving a con-trolvoltage which varies with said signal strength; and means for applyingsaid voltage to said control element to con-I trol the level ofselection, said voltage varying in such sense that said level ofselection becomes more remote from the peaks of said signal withincreasing signal strength.

3. In a carrier wave receiving system; a source of a modulated carrierwave signal; means coupled to said source for demodulating said carrierwave signal; a space discharge device having at lease an anode, acathode, a control grid, and an auxiliary accelerating electrodeinterposed between said cathode and said anode, said device having itscontrol grid direct coupled to said demodulating means; means havinganinput cir- 8. In an electrical system; a source of a signal cuitcoupled to said source for selecting a portion between predeterminedamplitude levels of the signal applied to its input, said selectingmeans being controllable so as to vary the level at which said portionis selected; and means for utilizing the D. C. potential of saidaccelerating electrode to control the level at which said selectingmeans operates.

4. In an electrical system; a source of a signal of varying strength;means having an input circuit coupled to said source for selecting aportion between predetermined amplitude levels of the signal supplied toits input, said means being controllable sov as to vary the level atwhich said portion is selected; space discharge means having a cathode,a control grid, and a plurality of output electrodes, said control gridbeing coupled to said source, and the D. C. potential of at least one ofsaid output electrodes varying with said signal strength; and means forutilizing the D. C. potential of the said output electrode to controlthe level at which said selecting means operates.

5. In an electrical system; a source of a signal of varying strength;aspace discharge device having a cathode, a control grid, and aplurality of output electrodes, said control grid being coupled to saidsource, and the D. C. potential oi' at least one of said outputelectrodes varying with said signal strength; a plurality of loadimpedances associated with said output electrodes; means having an inputcircuit connected across one of said load impedances for selecting aportion between predetermined amplitude levels of the signal developedacross said input, said means being controllable so as to vary the levelat which said portion is selected; and means for utilizing the D. C.potential of said one output electrode to control the level at whichsaid selecting means operates. l

6. In an electrical system; a source of a signal of varying strength;aspace discharge device having at least a cathode, a control grid and ananode, said control grid being coupled to said source; a load impedancein the anode circuit of said space discharge device; means having aninput circuit connected across said load impedance for selecting aportionvbetween predetermined amplitude levels of the signal developedacross said input, said means4 being controllable so as to vary thelevel at which said portion is selected; an impedance in the cathodecircuit of said space discharge device for developing a D.y C. voltagevarying with signal strength; and means for utilizing said voltage tocontrol the level at which said selecting means operates.

7. In a television system; a source of a composite signal comprisingvideo and "blanking components originally established so as to haveapposite polarities with reference to a blanking level; means having aninput circuit coupled to said source for selecting a portion betweenpredetermined amplitude levels of the signal supplied to its input, saidmeans being controllable so as to vary the level at which said portionis selected; means for deriving a control voltage which varies with saidsignal strength; and means for utilizing said voltage to control thelevel at which said selecting means operates so that said means selectsa portion of said input signal intermediate between said blanking leveland the peaks of said blanking signal for signal strengths within agiven range.

of varying strength; a space discharge device having an anode, a cathodeand a plurality of output electrodes, the D. C. potential of at leastone o1' said output electrodes varying with signal strength; loadimpedances associated with each of` said output electrodes; a secondspace discharge device having' its input connected across one of saidload impedances for selecting a portion between predetermined amplitudelevels of the signal supplied to its input, said second device beingoperated at low anode potential thereby to determine the magnitude ofythe selected portion, and said second device having an auxiliarycontrol element, the potential of which determines the level at whichsaid portion is selected; a source of voltage and a voltage divider pconnected thereacross; a connection from a point on said divider to thesaid electrode whose D. C. potential varies; a by-pass condensershunting a portion of said divider and means for applying thesubstantially D. C. potential developed across said portion to saidauxiliary' element to determine the level at which said portion oi' saidsignal is selected.

9. In an electrical system; a source of a signal of varying strength; aspace discharge device having at least an anode, a cathode and a controlgrid, said control grid being coupled to said source; a load impedancein the anode circuit of said space discharge device; a second spacedischarge device having at least an anode, a cathode, a control grid,and an auxiliary electrode, the signal developed across said loadimpedance being supplied to the control grid of said second device andthe said device being operated at low anode potential, thereby todetermine the magnitude of the signal portion selected by said seconddevice; a source of voltage and a voltage divider vshunting said source,a portion of said divider being included in the cathode circuit of saidrst device and being suitably bypassed by means of a condenser; andconnections from points on s aid voltage divider to the anode andcathode of said second space discharge device whereby the potentials ofsaid electrodes are varied in accordance with the current in the cathodecircuit of said first space discharge device thereby to determine thelevel at which said portion of said signal is selected.

10. In an electrical system, a source of a signal of varying strength,an electron discharge tube coupled to said source so as to receivesignals therefrom, means for operating said tube so that it transmitsonly a portion of an applied signal deined by lower and upper outputlimits, means for deriving from said system a control signalwhich'varies with signal strength, and means for applying said controlsignal to said tube so as to shift the eilective range of operation ofthe tube without substantially varying the output limits of the tube.

11. In an electrical system, a source of a signal of varying strength,an electron discharge tube coupled to said source so as to receivesignals therefrom, means for establishing substantially constant outputlimits of said tube, whereby said tube transmits only a portion of anapplied signal dened by said limits, means for deriving from said systema control signal which varies with signal strength, and means forapplying said control signal to said tube so as to shift the eiectiverange of operation of the tube without substantially varying the outputlimits of the tube.

12. In an electrical system, a source of a signal of varying strength,an electron discharge tube coupled to said source so as to receivesignals therefrom, said tube having a cathode, a control grid, an anode,and at least one auxiliary electrode, means for applying potentials tosaid grid and anode such that the effective operation of said tube isestablished between substantially constant lower and upper limits ofspace current now, whereby said tube transmits only a portion of anapplied signal defined by said limits, means for deriving from saidsystem a control signal which varies with signal strength, and means forapplying said control signal to said auxiliary electrode so as to shiftthe effective range of operation of the tube without substantiallyvary-l uration level, whereby said tube transmits a limited portion ofsaid signal, and means for effectively shifting the said operatingcharacteristic of said tube according to the strength of said signal.

14. In an electrical system, a source of a signal of varying strength, arst electron discharge tube coupled to said source so as to receivesignals therefrom, a second electron discharge tube coupled to said rsttube so as to receive signals therefrom, means for establishingsubstantially constant limits of output of said second tube, whereby thesaid tube transmits only a portion of the signal supplied thereto, meansfor deriving from said rst tube a control voltage which varies with thesignal strength, and means for applying said control signal to saidsecond tube so as to Vary the effective operating range of the tubewithout substantially varying its established output limits.

15. In an electrical system, a source of a sig" nal of varying strength,a rst electron dischargevv limits are established therefor, whereby thesaid tube transmits only a portion of the signal supplied thereto, meansfor causing the potential of the auxiliary electrode of said first tubeto varyk with signal strength, and means for causing the potential ofthe auxiliary electrode of said second tube to vary as a function of theaforesaid potential variation, whereby to vary the effective operatingrange of said second tube without substantially varying its establishedoutput limits.

16. In an electrical system, a source of a signal of varying strength, arst electron discharge tube coupled to said source so as to .receivesignals therefrom, a second electron discharge tube coupled to saidfirst tube so as to receive signals therefrom, each of said tubes havingat least triode elements and an auxiliary electrode, means for applyingsuch operating potentials to said second tube that substantiallyconstant output with signal strength, means for causing the potential ofthe auxiliary electrode of said second tube to vary as a function of theaforesaid potential variation, whereby to vary the effective operatingrange of said second tube without substantially varying its establishedoutput limits, meansfor derivingv a variable control voltage from one ofthe triode elements of said first tube, and means for applying saidcontrol voltage to a triodeelement of said second tube so as to furthercontrol the operation of the latter.

17. In a television receiving system of the type employing a compositesignal having video and synchronizing components established on oppositesides of a blanking level, a signal channel for receiving said compositesignal, a video signal channel coupled tosaid rst channel, asynchronizing signal channel coupled to said rst channel, an electrondischarge tube in said last-named channel, means for causing said tubeto have a predetermined operating characteristic with a substantiallyiixed saturation level, whereby the tube transmits a limited portion ofsaid composite signal dened by the cut-off and saturation points of saidoperating characteristic, and means for shifting the operatingcharacteristic according to the strength of said composite signalthereby to select a substantially constant portion of each synchronizingcomponent to the substantial exclusion of said video component.

18. In a television receiver, a source of modulated carrier Wavetelevision signals, a signal detector circuit, a synchronizing signalpick-ofi circuit coupled to said signal detector, said pickf oil"circuit being constructed and arranged to select only a portion of theavailable amplitude of the detected synchronizing signals, said portionbeing dened by both lower and upper cutoil limits, means for derivingfrom said receiver a control voltage` which varies with signal strength,yand means for applying said control voltage to said pick-o circuit soas to shift said limits in accordance with the strength of the receivedsignal, without substantially varying the output of said pick-olfcircuit. 4

. 19. AIn a television receiver, a detector for the modulated carrierWave television signals received thereby, a pick-offdevice consisting ofa space discharge device having its input coupled to said detector and`being operated at low plate voltage whereby it is caused to select aportion of the detected signal ybetween predetermined amplitude levelsfor synchronizing purposes, said devicehaving an auxiliary controlelement adapted, in accordance with its potential, to determine thelevel at which said portion is selected, means for developing a controlvoltage which varies with limits are established therefor. whereby thesaid the strength of said detected signals, and a vconnection betweensaid means and said control element whereby the level at which saidpick-oit device operates is controlled in accordance with said signalstrength.

2.0. In a television receiver, a detector for the modulated carrier Wavesignals received thereby, a space discharge device having a, cathode, acontrol grid coupled to said detector, and a plurality of outputelectrodes, a plurality of load impedances associated with said outputelectrodes, a pick-olf device coupled to one of said load irnpedancesand adapted to select a portion between predetermined amplitude levelsof the signal thereacross', said device being controllable so as to varythe level at which said portion is selected, and a D. C. connectionbetween one of said load impedances and said pick-oil device whereby thelevel at which said pick-oil' device operates is controlled inaccordance with the D. C. potential developed across said-last-namedload impedance.

21. In a television receiver, a. detector forthe modulated carrier wavetelevision signal received thereby, said signal comprising a carriermodulated by a composite signal including video andsynchronizingcomponents originally established so as to have oppositepolarities with reference to a blanking level, a pick-oil device coupledto said detector and adapted to select a portion of the detected signalbetween predetermined amplitude levels for synchronizing purposes, saiddevice being controllable so as to vary the level at which said portionis selected, means for developing a control voltage which varies withthe strength of said detected signal, and a connection between saidmeans and said pick-off deviceA whereby said pick-oir device operates toselect a portion of said signal intermediate between said blanking leveland the peaks of said synchronizing signal component.

22. In a television receiver, a detector for the modulated carrier wavetelevision signal received thereby, an electron discharge tube coupledto said detector so as to receive signals therefrom. means for operatingsaid tube so that it transmits only a portion of an applied signaldefined by lower and upper output limits, said transmitted portion beingemployed for synchronizing purposes. means for deriving from saidreceiver a control voltage' which varies with signal strength, and meansfor applying said control voltage to said tube so as to shift thee'ective range oi' operation oi' the tube without substantially varyingthe output limits of the tube.

23. In a television receiver adapted to receive a signal of varyingstrength, a synchronizing signal pick-oil' device comprising an electrondischarge tube coupled to said receiver so as to be supplied with saidreceived signal, said tube having a predetermined input-output operatingcharacteristic and a substantially fixed saturation level, whereby saidtube transmits a. limited portion of said signal to be used forsynchronizing purposes, and means responsive to the strength of thereceived signal for effectively shifting the operating characteristic ofsaid tube to thereby vary the level at which said portion is selected.

R. APPLEGAR'I'H, J R.

